1. Field of Invention
This invention pertains to a sensor for the detecting the Earth from a satellite, and more particularly to a system for detecting the horizon of the Earth to determine the satellite's attitude.
2. Description of the Prior Art
As a satellite is orbiting the Earth, its attitude must be checked periodically to determine the pitch and roll of the satellite. This task is normally accomplished using Earth sensor systems having IR detectors which monitor the surface of the Earth and determine the position of its horizon by comparing the radiant contrast between the Earth and space. However present systems are very susceptible to errors when the Earth's surface presents variations in reflectivity or radiance caused by seasonal or geographic effects. There are several prior schemes used to compensate for this variation however, all of these schemes either require that the radiance variation be known or use a variable threshold scheme.